1.FIELD OF THE INVENTION
This invention relates to improvements in power circuits, and, more particularly, to improvements in inverter circuits, and still more particularly to an inverter circuit having modulating control to produce distortion and modulation free output.
2. DESCRIPTION OF THE PRIOR ART
In the field of power circuits, inverters are frequently employed to deliver AC signals from a DC input. Sometimes in certain power train applications, it is found to be most efficient in some applications to produce an AC output at a different level and/or frequency from an AC input by first rectifying the AC input, then applying the rectified voltage to an inverter to produce an AC output at the desired voltage level. Often times, however, when such rectifying and inverting techniques are used, inverters typically employed produce a great deal of amplitude modulation distortion in their outputs, especially when the rectified voltage is lightly filtered.
As an example, in certain aircraft and military applications, a power requirement is frquently encountered in which an AC voltage of 400 cycles is needed. In the past, this has been accomplished by first rectifying a standard 60 cycle power voltage and then filtering the resulting rectified DC voltage. Thereafter the rectified and filtered DC voltage is applied to an inverter which operates to produce a 400 cycle signal at the desired voltage. Not uncommonly, however, very large filter capacitors are required to remove the unwanted harmonics and modulation components from the rectified signal, filter capacitors on the order of 10,000-20,000 microfarads per 40 KW of output power typically being required for this purpose. More particularly modulation subharmonics which are produced are, as is well known in the art, a sum of multiples and submultiples of the fundamental frequency and the modulation frequencies, each modulation subharmonic being at a particular amplitude. The filter, used in the past to remove these modulation products, has therefore been primarily directed at reducing the respective amplitudes of these modulation products. Furthermore in many frequently encountered applications, two three phase inputs are applied to a power train of the type described to produce a three phase output of desired voltage and frequency with one of the three phase inputs typically being thirty degrees out of phase with the other to permit ease of processing using standard, well known techniques.
What is needed is a technique for producing an inverter output with reduced, or preferably no amplitude modulation, especially under load conditions, and which has a significantly reduced requirement for large filter capacitors, and which can, if desired, be used in conjunction with available plural three phase inputs of known phase difference.